1) We had ran into some query optimisation issues with Mysql that I blogged here http://neopatel.blogspot.com/2012/04/mysql-subquery-multiple-columns.html
2) Just ran into a 4 byte unicode issue with Mysql where utf8 with mysql wont support 4 bytes characters and apparently you have to use utf8mb4 for it, how stupid of Mysql. Will write a separate blog post for it.
But as far as performance is concerned its rocking so far. On 20th we did the deployment and as soon as the node went live with Mysql you can see immediate drop in load avg consistently for this week. This is because most of the processing that was done by in memory db on app node is now transferred to Mysql. But in second graph you would see that Mysql is sitting ducks and its no where close to app node load avg. Now this is with 3 nodes pounding, we would add 10+ nodes to start with on each Mysql and then consolidate later as I want to start conservative.
|App Node Load Average|
|Mysql server load average|